1. Field of the Invention
This invention relates to motor driven cylinders for use as prime movers and more particularly to such cylinders as utilized in lifting apparatus, or the like.
2. Prior Art
Motor-driven cylinders have, for many years, been utilized in lifting apparatus, for example tables for lifting heavy loads or in elevators. It is apparent that, should the motor-driven cylinder fail, it would be dangerous to the load being lifted, for example to the occupants of an elevator. Thus, all such systems have been equipped with safety devices to prevent the object being lifted from falling under the force of gravity when the lifting apparatus fails, for any reason. It has been the practice, in the past, to have the safety equipment separate from the drive cylinder itself. Such an arrangement has made the lifting apparatus costly and unnecessarily large in size. One such safety device involves externally and internally threaded and cooperating propulsion screws, coupled to the motor drive, those propulsion screws being such that they can be automatically stopped. For example if the braking system on the drive motor fails to operate or the drive motor fails to produce normal torque, the vertical movement of the lifting apparatus is automatically stopped by the load in the direction of propulsion to achieve the desired safety. However, the screws which will automatically be stopped when the motor fails to produce adequate torque, have a low efficiency of about 30%, requiring under normal operation, a drive motor output that is three times that of an ordinary drive motor. Because the screws suffer an increased frictional loss, they are subjected to a high temperature rise and cannot be used frequently. The increased output required of the drive motor makes the apparatus unduly large in size and unnecessarily expensive.
Therefore it is an object of this invention to provide an improved motor-driven cylinder with inherent fail-safe capabilities.
It is a further object of this invention to provide, for lifting apparatus, a motor-driven cylinder which is of minimum size and expense and still exhibits the safety features required for safe use of the motor-driven cylinder.